%I #42 Jul 07 2023 19:04:41
%S 1,2,3,2,3,3,4,4,3,3,4,4,5,5,5,5,6,6,7,7,7,7,8,8,7,7,7,7,8,8,9,9,9,9,
%T 9,9,10,10,10,10,11,11,12,12,12,12,13,13,12,12,12,12,13,13,13,13,13,
%U 13,14,14,15,15,15,15,15,15,16,16,16,16,17,17,18,18,18,18,18,18,19,19,19,19
%N a(n) = pi(n) - pi(sqrt(n)) + 1.
%C A well-known application of the principle of inclusion-exclusion used in sieve methods.
%C Number of numbers less than or equal to n and coprime to the product of the primes less than sqrt(n), i.e., to A104588(n). - _Lekraj Beedassy_, Mar 17 2005
%H G. C. Greubel, <a href="/A079715/b079715.txt">Table of n, a(n) for n = 1..1000</a>
%F a(n) = pi(n) - pi(sqrt(n)) + 1 = A000720(n) - A056811(n) + 1 = A056812(n) + 1.
%F a(n) = Sum_{k=1..n} mu(k)*floor(n/k) where each prime factor of k is <= sqrt(n). [Corrected by _Steven Foster Clark_, May 03 2023]
%t Table[PrimePi[n] - PrimePi[Sqrt[n]] + 1, {n, 1, 100}] (* _G. C. Greubel_, May 13 2017 *)
%o (PARI) for(n=1,100, print1(primepi(n) - primepi(sqrt(n)) + 1, ", ")) \\ _G. C. Greubel_, May 13 2017
%Y Cf. A000720, A056811, A056812, A104588.
%K nonn
%O 1,2
%A _Benoit Cloitre_, Feb 16 2003
%E Edited by _N. J. A. Sloane_ at the suggestion of _Andrew S. Plewe_, Jun 12 2007
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